I have just the one length hub right now. It can not be much shorter than it is because then there would not be enough threads for the prop bolt. As it sits right now the shortest it could be is 5 inches. On most 50cc size planes, the length will not be a problem. Most seem to need closer to 6" or more.

I am trying to find some info on the Echo on a pipe. Nothing yet, but I will modify a header if need be and try it.